Polyisocyanate mixtures containing alkoxysilane groups have been known for some considerable time. Products of this kind, which as well as the isocyanate group contain a second reactive structure, in other words a structure capable of crosslinking, have been used in the past in various polyurethane systems and polyurethane applications with the aim of obtaining specific properties, such as, for example, to improve the adhesion, chemical resistance or scratch resistance of coatings.
By way of example, WO 03/054049 describes isocyanate-functional silanes, prepared from aliphatic or cycloaliphatic polyisocyanates of low monomer content and from secondary aminopropyltrimethoxysilanes, as adhesion promoters for polyurethane hotmelt adhesives.
According to the teaching of JP-A 2005015644, as well, polyisocyanates or isocyanate prepolymers that are modified with N substituted, i.e., secondary, aminopropylalkoxysilanes can be used to improve the adhesion of adhesives and sealants.
EP-B 0 994 139 claims reaction products of aliphatic and/or cycloaliphatic polyisocyanates with substoichiometric amounts of alkoxysilane-functional aspartic esters, as described in EP 0 596 360 as coreactants for isocyanate-functional compounds, and optionally with polyethylene oxide polyether alcohols, as binders for one-component, moisture-crosslinking coatings, adhesives or sealants featuring accelerated curing.
Reaction products of aliphatic and/or cycloaliphatic polyisocyanates with substoichiometric amounts of alkoxysilane-functional aspartic esters or secondary aminoalkylsilanes are also described in WO 02/058569 as crosslinker components for two-component polyurethane adhesion primers.
EP-B 0 872 499 describes aqueous, two-component polyurethane coating materials which as a crosslinker component comprise compounds containing isocyanate groups and alkoxysilyl groups. The use of these specific polyisocyanates leads to coatings having improved water resistance in conjunction with high gloss.
Hydrophilically modified polyisocyanates containing alkoxysilane groups, which are therefore easier to emulsify, have likewise already been identified as crosslinker components for aqueous two-component coatings dispersions and adhesives dispersions (e.g. EP-A 0 949 284).
To improve the scratch resistance of solventborne thermosetting two-component automotive PU clearcoat and topcoat materials, recent times have seen proposals for reaction products of aliphatic and/or cycloaliphatic polyisocyanates with N,N-bis(trialkoxysilylpropyl)amines as a crosslinker component (EP-A 1 273 640).
WO 2009/156148 describes reaction products of isocyanate-functional compounds with substoichiometric amounts of mercaptosilanes as crosslinking gents for automotive clearcoats in OEM production-line finishing or in automotive refinishing.
Common to all of these polyisocyanate mixtures containing silane groups is that they are prepared by proportional reaction of unmodified polyisocyanates or polyisocyanate prepolymers with organofunctional silanes that contain groups reactive toward isocyanate groups, examples being mercapto-functional silanes, primary aminoalkylsilanes, secondary N-alkyl-substituted aminoalkylsilanes, or alkoxysilane-functional aspartic esters.
Such modification, however, leads inevitably to a reduction in the average isocyanate functionality relative to that of the original polyisocyanates employed, the effect of this increasing in line with the target silane content of the reaction product. In actual practice, however, the desire is actually for polyisocyanate crosslinkers with as high as possible an isocyanate functionality in the aforementioned applications, such as coating materials or adhesives, for example, in order to achieve a high network density.
Furthermore, as the degree of modification—that is, the silane group content—goes up, there is also a drastic increase in the viscosity of the products, owing to the thiourethane groups and, in particular, urea groups that are introduced into the molecule; for this reason, the silane group-containing polyisocyanates known to date can generally only be used in dissolved form, using considerable quantities of organic solvents.
An exception in this regard is represented by the polyisocyanates containing allophanate groups and silane groups that are described in EP-A 2 014 692 and EP-A 2 305 691, that are obtainable by reaction of hydroxyurethanes and/or hydroxyamides containing silane groups with excess amounts of monomeric diisocyanates; in spite of high isocyanate functionalities and high silane contents, these polyisocyanates have comparatively low viscosities. However, preparing these specific silane-functional polyisocyanates is very costly and inconvenient, and involves poor reproducibility, owing to the low stability of the hydroxyurethane and/or hydroxyamide intermediates that have formed from aminoalkylsilanes with cyclic carbonates and/or lactones, respectively.